This invention relates to a method for improving emulsion paint. More particularly, the invention relates to a method for improving the coating property and the storage stability of emulsion paint.
The emulsion paints have been used as indoor and outdoor wall paints and so forth, and the paints of this type have come into notice because they have several advantages that, as compared with the ordinary organic solvent type, the organic solvents can be saved, evaporation of the organic solvent vapor in drying step is little and air pollution is therefore little. However, the coating property and the storage stability of the emulsion paint are somewhat inferior to those of the organic solvent types, so that the utility of the emulsion paint could not be wide enough for baking type paint.
The emulsion paints are dispersions mainly containing emulsified resins and pigments, and therefore the emulsion paints have a high thixotropic property and the viscosity is liable to become low. As a result, the painted film has the disadvantages that the flowing property is poor and the desired film thickness is unobtainable. In order to give a good coating property to the emulsion paint, the thixotropic property of the emulsion paint must be made low and the viscosity of the emulsion point is to be regulated within a proper range, for example, about 60 to 100 seconds No.4 Ford. cup at 25.degree. C, and for this purpose, there are well known the following two methods.
One method is to add a small amount of high molecular water soluble resins to the emulsion paint, and as such resins, methyl cellulose, carboxymethyl cellulose, polyacrylamide, sodium polyacrylate and the like are widely used. According to this method, the viscosity of the emulsion paint can be surely increased, however, there is no effect for lowering of the thixotropic property of the paint, and what is worse, the water resistance of the dried paint film is decreased due to the addition of the water soluble resin.
The other method is to add a small amount (0.1 to 1 part by weight to 100 parts by weight of paint composition) of some surface active agents such as anionic, cationic, nonionic or ampholytic surface active agents to the emulsion paint. The thixotropic property can be reduced by means of this method, however, this method also has a disadvantage that the water resistance, moisture resistance and chemical resistance of the coated film are made worse. Therefore, lowering of practical properties of the paint film can not be avoided through the above-mentioned conventional two methods.